Infectious RSV was produced by the intracellular coexpression of five cDNAs encoding: (i) a complete version of RSV replicative intermediate RNA, and (ii) the four RSV proteins identified in other work as being necessary and sufficient to produce nucleocapsids competent for transcription and RNA replication (accompanying report). The ability to produce RSV from cDNA establishes the first available method for the direct engineering of infectious RSV. One important use will be to facilitate and extend the characterization and development of a live attenuated RSV vaccine based on existing candidate vaccine strains. Specifically, the mutations responsible for desired phenotypic characteristics (attenuation, temperature-sensitivity, cold-adaptation, small plaque size, host range restriction, etc) of these existing viruses can now be directly identified and characterized by their individual insertion into the "wild type" genome. Desirable mutations from this menu can be mixed in various combinations to fine- tune phenotypes. This also will make it possible to modify vaccine virus to accommodate antigenic drift in circulating virus. A second important use will be to explore new possibilities for improving live attenuated vaccine viruses. New types of attenuating mutations probably can be developed. The level of immunity associated with natural infection might be improved quantitatively or qualitatively in a recombinant virus by the inclusion of cytokine genes or additional T cell epitopes, by ablation of possible epitopes associated with reactogenicity, or by other manipulations. A third important use will be for exploring the roles of individual viral genes in virus replication and pathogenesis.